Two-way telephone transmission system



Aug. 3, 1943. K. G. VAN WYNEN 2 TWO-WAY TELEPHONE TRANSMISSION SYSTEM Filed Feb. 25, 1941 INVENTOR K. G. VAN W VNE N /27 B) MKW A T TOQNE --ance the two-wire line.

Patented Aug. 3, 1943 TWO-WAY TELEPHONE TaANsr/irssi'oN c SYSTEM Kenneth G. Van y amsy, El. 352-, assigiia to Bell Tclplifi Laboi'atories, Incorporated, New York; N. Y., a corporation in New York Amlieation Februae 2 5, 19 Serial 38 8; 428 l a 7 cases. 0 v 179;1753) t i The invention relates to a two-way telephone transmission system and particularly to' a sys- 'tem of this sort in which a four-wire toll telephone circuit is associated at a terminal switchboard with a connection extending to a local subs'cr'ibers line. The four-wire toll circuit provides two two-wire circuits respectively used for transmission in opposite directions and the two-wire circuit to the subscriber circuit transmits in both directions. i i

It is usual to connect the four-wire circuit to v the two-wire circuit by means of a special type of transformer known as a hybrid coil. In such arrangements for efficient and satisfactbry signal transmission an artificial line or network is provided in association with the hybrid coil to bal- Where the fcur wire circuit terminates in a toll board, it inay be there connected with a variety of tWowire lines of different impedances in establishing a connection.

to a subscribers line. Because the siibscribers loops 'may be of different impedances and the connection may include a'variabl number of inter'oflice trunks, an artificial balancing line which will provide an accurate balance forone ccnnection may not be sufiiciently accurate for other connections and may, thus result during, voice transmission over the system in the production of undesired'unbalance echo currents in the fourwire circuit. This condition heretofore has been taken care of by providing'a Compromise artificial balancing line having an impedance equal to the average impedance of the two wire lines with i whicn conne'ction is to be made and by the provision of other means such as echo suppressors to compensate for unbalance conditions.-

In accordance with one embodiment of the present invention, a number of artificial balancing lines, so-calle'd balancing networks, of varione selected impedances, simulating those of dif ferent-ones of the range ;of two-way lines to be connected to the four-wire circuit, are provided together with means to automatically compare the balancing networks with the line tOfbe balencode and to bring about the association with the hybrid coil of the particular balancing net-j :wcrk which gives thetmcst advantageous balance in any" given connectionl Also, in accordance with the invention, the selection and con n'ection of the optimum balancing network takes place in the early stages of setting up theyconnecti'on and before the c'onnection is piacedwin I final condition for signal transmission.

In the drawing there is shown aportion of the usual four-wire' t'oll circuit having ah' olltgoing I when inserted in the jack J, .asso'ciates the par- Any usual form of this apparatus may be u ed,

stance. J the line he is associated with the line windings line Ll including one-way amplifier AI, and an incoming line L? including one way amplifier A2,

adapted to be connected thrbugh. .the usual hy-f brid co'il H to a subscribers line L3. the

subscribers line L8 may include in addition to the subscribefs loop intermediate. connecting links such as interoii'ice trunks, it is necessary to associate with the hybrid oc-il'an artifici-aibhlarising line or network which is a close approximation in impedance to that of the local line L3 Whichit is desired to balance; The subscribers line L3 is shown as terminating in a plug P which,

ticular line with the toll circuit and the ballancing network selected. in any particular in- When the plug P is inserted in the jack of the hybrid coil H through the back contactsof the relay 16!. It is, therefore, necessary. to connect across conductors I and 2 a balancing network which gives the nearest apprcximationto an exact balance. Inasmuch as it is impracticable to provide. an infinite number of, such networks, a sufficiently good approximation may be obtained by providing a number of such networks which have impedances equal to the yarious line conditio'nsmost likely to be encountered in practice and to associate with any particular connectionthat one of such networkswhich provides the nearest approximation to a balance under any givencondition. p

"At B there is shown a Wheatstone bridge i 31 vided with the usual resistance armsRl and R2, a source oi audio frequency, tone T connected across one diagonal of the bridge and an amplifier filter -rectifier combination A 3; F and RF connected across the other diagonal of the bridge.

the arrangement being such that the voltage produced by the tone appears as a direct current potential on the conductonl32; the voltage of which depends upon the degree of unbalance of the bridge under any given set of conditions. As the line L3 is connected in on arm of the bridge and a particular balancing network under test is connected in another, this directcurrent potential is ameasure'oi the degree of unbalance between the subscribers line and the particular network under test. 7 a i i t.

For n as iliustrationin describing the operational? the system, four: balancing networks are shown; Ni", N3 an'dzN i', of various iinpedance's which may comprise number of resistance", inductance anti capacitance elements "in such circuit combinations and-1c]? such relative the contact lih'lc'is to a contact which is values as to simulate both in magnitude and phase different lines of those which may be connected to the four-wire circuit, although it is understood that a larger number may beused,

depending upon the range oi impedances which it is expected to encounter in the lines to be balanced. v

The test for the optimum balancing network takes place before the connection is cut through tothe subscriber and conversation takes place and, in order to reduce the time to the minimum for making the test and selecting" the desired network, the selection is made by means of fastoperating relays, as will appear fromthe follow:

ing description of the operation. 'While there is a large number of these relays involved in the system disclosed and a description of their operation will be somewhat involved, it will be understood that the operation takes place at a very rapid rate in order that there shall be the mini- .mum amount =01 delay in setting up the actual connection between the subscribers.

In generahthe test and selection are accom plished byassociating in'succession with the Wheatstone bridge a number of balancing net- .Works, and storing a charge on reference conxdensers ill} and iii which serve as a measure of. the difference between the impedance of the line L3 and the successive balancing networks which maybe associated with the bridge. These 7 two potentials are then compared through a com- .parison circuit including the vacuum, tubes Hi5 and l H and thedii-Ierential relay 8 it, this com- ;parison being repeated with the association of Various networks with the bridge untila network is found which gives the best balance and this network is finally, connected to the hybrid lay are referred to as front contacts and those operated by therela in;unenergized condition as back contacts.

. Referring now to detailed operation of the testing and selecting optimum ne works, 'it wiil be notedthat this operation is initiated when the operator inserts the-plug P into the jackJ. a As a result of this operation the startrelay ltd is operated through the sleeve of the iackwith the following result. Relay tilt, in operating, places ground through front contact iota on the'conductor leading to relay lilii through conductor I28, back contact lZSa, conductor its, back contact ofrelay l l l, to relay ass, to battery, operating relay lot. It is here noted that relay lot which as shown is slow to operate, ca ses the operation of slow release relay 5 i l which, in operating,- opens theenerglZin-g circuit for relay lili Thus, these relays, which in effect form a timing orpulsing device, will continue to operate and release While the selecting and testingoperation to be described takes place.

At front contact will) a circuit is established for relays it! and IE2 in parallel through contact liltb, conductorltil, back contact 22% to ground, thus operating relays ltl and Relay liil, whenoperated, connects line L3 to one arm of the bridge 3 and relay I52, when operated, disconnects conductors l and 2 from the hybrid coil H.

'A locking circuit for relay lot is closed at front contact which may be traced from the contact through *"iductor ltl, back contacts i260; ittf, 538g, i through re iilS to battery. It will beobserved that network Ni is norrnally connected through the 11-4) contacts of relays let, 5% and E03 to the other arm oi the bridge and the comparison between the impedance of the network Ni and the line L3 takes place as soon as the relay is operated. As soon as operated, there will be applied to the c nductor I32 a rectified potential which'represents the degree of unbalance of the bridge B, or, in other words, a poten ial wh represents the difference in balance between the network Ni and the line L3. Through the make contact H330. the conductor 53?: is extended to conductor 33 and causes the charging of the condenser H9 to the potential placed on conductor l32.

.It will be recall d that relay W6 is shown as a slow-to-cperate relay and hence does not operate until after. the condenser H8 has 585i operates, a circuit is establishedicrrelay it? which may be traced from battery through work N2 is connected to the bridge B and the network N! is disconnected therefrom.

The relay lil'l, when operated, closed a locking circuit for itself through its contact lttf, back contacts ma ll9f,jl26c, front contact ltscto ground. The previously traced locking circuit for relay I83 through contact till is opened when relay 191 operates, and relay H33 releases. Also as a result of the operation of relay lil'l, conductor l32 is extended through front contact lll'ld, back contact 3d, back contact lilSe, back contact 109d, conductor i3 1 to condenser l l2. Under these conditions condenser H2 will accumulate a charge which is a measure of the unbalance between the line L3 and the network N2 which is now connected to the bridge 13 through the operation of the relaynlll'l. It will be noted that the relays ltl, lliSand I89, when operated, successively connect the:netw.orks.N2, N3 and N4 to the bridge through their .ab contacts and disconnect the previously connected network.

-. We'nowhave a charge on. the condenser 'l is which represents the degree of unbalance between the line L3 and the network Ni and on condenser I H! a charge representing the degree of unbalance between the line L3 and the network N2; These charges are now compared to'determinewhich of the two networks most nearly approximates a balance, or, in other words, which network is better for balancing purposes. i. r

When relay Ill? operated, a circuit wasv also established for relay MS as follows: from battery through relay H9, conductor l35, back contact I651), conductor l36, back contact l@9g,'back contact 38h, front contact 5679, to ground. With the relay I H? operated the differential relay l 58 is associated with the'plat'e'ci-rcuits of the ivacuum tubes llii and Ill. These tubesare-of the threeelectrod'e high vacuum type.- By means of;the biasing battery it! the grids are negatively biased to an extent which insures that the potential impressed on the grid by the charges on condensers H0 and .l I 2 is in any case insufficient to make the grids positives Inorder to prevent the: discharge been charged, as previously described. When relay the event that the varistors should creel;

balancing network N2 against that'wi aseaoei or these condensers before the charges-are compared; as will presently appear, the plate battery of the tubes i'sconnected through retardcoils of high impedance l39' and M3 to the platesof the "tubes. It will also be noted that with relay H8 operated the windings of'the diiiferential relay H3 are in shunt with these retard coils and, as they areof low impedance relative to that of the retard coils, as soon as this connection is made current will flow through the windings of relay 118 which is proportional'to the potentials on" the grids of the tubes and, consequently; proportional to the potential onthe condensers t It and H2,

'tains the armature will not be attracteds Varis'ftors l2fi'and'l2l are includerl'in shuntwith the windings of the differential relay to protect them in the event-that" a very large plate current should flow'in one and not in the other due to a large diiierence in the charges ontlie condensers H and H2. Resista'nces"arepincluded in series with the varistors to limit the; flower current in down.

Assuming: that the charge on ccnd less thanthe charge on con'denser lld, in ating that balancing network N2 is better than its the circuit proceeds to compare the balance with E1 balanc- "ing network N3 in the following manner. jUnder the assumed condition relay MB is operated,

establishing an operating circuit: for relay "from battery through relay Hi l; conductor Ml,

contacts of relay 6 lihback contact ifiddto ground.

3 As soon as relay lad is operated it operates relay 105 which opens the energizing circuit for relay its: Relay ltd is slow to release in order that the relay 1% may be completely operated before the energizing circuit of relay tilt is opened at the hack contact H6501. The closingof front conthe resistance Relay through operated contact lti a operates relay H3 over a circuit ex tending :from battery through relay H3; make contact (lillh, and the fro-ntf contact I555 to ground. 'Q j With relaylr'il operated'and relay HBoper- 'ated, conductor E32 is exte nded through the contact 33a, ltl'd front, lliid front to condenser iltl.

,fBearing in mind that with relay l Sl operated ar- 'ftificia1lineN2is now connected in'thebridge B, t e -'charge' accumulated on condenser lit repi'esents the degree of unbalance between network N2 andtheline' LS. It may be well to pointlcut here that each of thefrelays i it, i Mi H when operated close a locking circuitextending through its ownfront ccontact and contact leiic. When relay H35 releases, as above described, upon the next operation of timing relay .196, ground is placed eitheron the front or backcontact 1931;. Under the conditions assumed, if the front ccn- .=1 s act;..is. made nothing happens as relay; Hill is locked up and relay lcilhas not-been operated, hut it is 'estabwhen the back contact is made a siren lishe-d .for relay 93 from battery winding of the re 'make' contact ough the 3 i c, back c'ontact 'i'iSaQbacl: contact lets to ground. Relay tilt-is opcratedand causes the release of relay f l filsince' it Opes the previously traced locking conductor 53 to condenser 152. 1

-With relay 1B3 operatedand' it? released, balancing network N3 is. connected tothe bridge throughthe a--b contacts of relay lot and the wb contacts of relay Ill. as soon as this connection has been made conductor 532 is extended through back contact it'd, front coir tact we, back contact l 14d, back contact 109d. conductor 54 to condenserllfi. Therefore condenser 4 I2 is charged to potential whichtrep- 10 resents the degreeofunbalancerbetween-line L3 and balancing network -We now have the two-condensers H0 and HZchar ed to potenconnects the relay H3 in operativerelationito the two vacuum tubes M8 and l l1 and the cornparison takes p-laceas before described. It will be recalled that was assumed thatthe second network; N iaffordeda better be. ice to the l,

L3 than network NE and the action of the c cuit in comparing networkNE with N3 has been on of to be described. In order to show the cpera the circuit when the last network an-p less favorable than the network pi e/rot ed, it will now be assumed that netwc better than N3. In this case he difiereniral lay. H8 fails to operate. It is thc refore'necessary to' retain the charge representing the balance of N 2, or.the better network. on condenser H0 and to discharge-the condenser H2. fails to operate under the conditions the next closure of'the timing relay contact c front will cause the operation of the l over a circuit" which maybe traced vibrattery through the relay me, make contact lii'f, make contact l 236a, back contact H350. to ground. Under these conditions condenser charged through a -grounding r through front contact time beiorethat leases due to the operation of relay I discharge circuit may be traced resistance i225, contact W80 front; F

As the locking circuit ofrelay tiifiincluded a -contact of relay IE9, which is now open, relay Hid-releases. Also the previously traced cii i for relay H9 is opened through hack [89g preventing false operation of relays" H8 ait this time. With relay 8% opera network N 6 is connected to thebridge B through 1 the Cib contacts of the relay and under this condition the conduct ""td through contact 553a back, liiid. back, c back, it -front, l'l5d back to conductor ltd and thence to condenser H2. Also-as a result of the'operation of relay H59 a circuit for relay I I9 is extended to conductors IdE and lSfi through ='-ta'ctlilac, back contact i' Si} back c utoground, and the circuit is set up to r as test as before an to compare the ch on condensers!!!) and H2; If No is betterthan N2 as indicatedb-y the 1test,-re1ay H3 operatesas before ca mg the operati on-cf 'relay HM and the consequent opl ation of relay Hi5 anddischarges condor 4 "ers l lfland H2. With relay ltfireleased, which *takes place immediately after its energization 'owing 'to the release 'c'rrelay ltl; on the next the relay ce is released. 7 connected through the release of the relay lfil,

closure of the back contact of Iilfia back of the timing relay, relay H19 operatesrelay i126 which is the last relay in the chain and is operated over a circuit which may be traced from battery through the relay, i26, contact 1092 front, ifiliia back and H150. back to ground. Relay E26 operated removes the ground over which relays liii and I92 were previously energized causing these relays to release. The release of relay [62 con nects the networkN i, which has beenselected asv thejbest for the purposa'to the right-hand winding ofgthe hybrid coil H and the release of the relay liil connects the line. L3 -,to the lefthand winding of the hybrid coil, and conditions are now such that the talking connection may be cut through Incidentally, the circuit for the timing relays H33 and Hi is'opened at contact IZta and any operated relay of the series I09 which may be locked up through the contact c of IE6 is released. However, relay H which was operated when relay H39 was operated locks up through its contact IiE'ec through conductor Hi to contact H160 and maintains the, network N4 connected to the conductors i and 2 after The bridge B is dieand the entire testing system is restored to nor mal and made ready for use when the next connection is established. 7

Thus far no mention has been made of the re lay i2? and its function since this function may be omitted without affecting the above-described operation of the testing system. Relay I21 may be connected, however, in bridge with resistancef i233 when it may be desired to terminate the testing if a network isfound that is sulfa ciently good for practical purposes. This condi- V tionwill be indicated by a small current flow in the circuit including the resistance H23 indicating a slight degree of unbalance between the line L3 and the network under test. However, if this condition does not obtain and a substan tial current passes through the relay. iii, and

the resistance I23, the relay '52? will remain energized and the testing will proceed as described, If, however, the relay E21 releases its armature when relay H9 is energized, relay Q26 will be 3 energized and will stop the testing operation and cut the selected network through to the hybrid coil H. Relay i2? is a fast-operating relay, al-

though marginal; whereas relay lie is slow to operate so that the relay 12? will ordinarily oper- I 23 afi'ects the current through the lowerwinding v of said relay, thereby maintaining a balanced circuit relation.

Various modifications of the circuits illus- I trated and described which are within the spirit, and scopeof the invention will beapparent topersons skilled in the art. For example, the ,hy-

'brid coil bridge at oneterminal of the four-wire telephone circuit maybe employed to provide, in addition to its usual function, that of the auxiliary Wheatstone bridge in themeasuring and network selecting circuit, so as to allow elimination of the latter bridge. In that case, relays ing one arm of the hybrid coil bridge, would be employed to connect the input of amplifier A3 and the source of tone T respectively to one of the two diagonals of'the hybrid coil bridge, and

' bridge circuit having line and balancing network to connect the balancing networks N l, N2, etc., in sequence to the net side of the hybrid coil rorming another arm of the hybrid coil bridge,

and to disconnect the measuring circuit and the source of tone from the hybrid coil bridge when the balancing network of optimum value is selected and connected to thenet side of the hybridge arms and the input and output of said paths at said terminal respectively connected in the other two opposing arms, a plurality of twoway signal lines adapted for selective connection to said line terminals, and means to reduce signal echoes for a signal connection between said four-wire circuit and any one of said lines, comprising automatic means responsive to the connection of a selected line to said line terminals totest the line to determine an electrical characteristic thereof and to provide a balancing network of a tolerable matching electrical characteristic connected to said network terminals.

2'. The combination of claim 1 in which the I determination of the electrical characteristics of the connected line and the provision of the balancing network of matching. electrical characteristic in connection with said network'terminals are made before the signal connectionbetween said four-wire circuit and the selected line is established.

3. In combination in a two-way telephone system, a four-wire telephone circuit including two oppositely directed telephone transmission paths and at one terminal of the circuit a'hybrid coil terminals'in' two opposingbridge arms,.the input and output respectively of said two transmisconnected line to determine an electrical characteristic thereof, and to select and connect to said network terminals one of said balancing networks having a tolerable matching electrical characteristic.

4. The combination of claim 3 in which said automatic means comprises means for comparing theconnected line in sequence with a number of said balancing networks, means for discarding the networks providing insufficient balance for the connected line and for selecting and conoperatively responsive to the connection of the line L3 to the line side-of the hybrid coil form:

necting to said network terminals, one network which provides a tolerable balance.

5. The combinationof claim 3, in which each of said balancing networks simulates the impedance characteristics both in phase and magnitude of a difierent one of saidlines, and said and means to select and connect said one balautomatic means comprising an auxiliary bridge circuit, a source of current connected across one diagonal of said auxiliary bridge circuit, means responsive to the connection of a selected. line to said line terminals to connect said selected 7 line in one arm of said, auxiliary bridge circuit,

to connect said balancing networks in sequence in another arm of said auxiliary bridge circuit, to determine the unbalance of that bridge for each balancing network connected therein, to compare the determined unbalances for the'several balancing networks, to discard those net works providing insuflicient balance of said connected line, and to select and connect to said balancing network terminals of said hybrid coil bridge circuit one of said balancing networks having an optimum impedance matching characteristic. V

6. The system of claim 3 in which said automatic means comprises an auxiliary Wheatstone bridge circuit having two equal arms of predetermined equal value and two other arms, a source of current connected across one diagonal of said auxiliary bridge circuit, relay means responsive to the connection of said selected line to said line terminals of said hybrid coil bridge circuit to connect said selected line in one and said balancing networks in sequence in the other of said other arms of said auxiliary bridge circuit, means for storing energy proportional to the unbalance current of said auxiliary bridge circuit for each balancing network connected therein, means to efiectively compare the amount of stored energy for the several connected balanced networks to determine which one provides the least amount of stored energy indicating that it provides the best balance for the selected line,

7. In combination in a telephone system, a

four-wire telephone circuit including a hybrid coil set having line and balancing network ter-' minals, at a terminal thereof, a plurality of different two-way telephone lines terminating at said terminal, means for selecting and connecting to said line terminals any one of said twoway lines, and means to reduce echoes in said circuit during telephone transmission between said circuit and any connected two-way line comprising a plurality of balancing networks each simulating the impedance characteristic both in phase and magnitude of difierent ones of said lines, a Wheatstone bridge circuit, a source of current connected across one diagonal of said bridge circuit, means responsive to the connection of the selected line to said line terminals for connecting said selected line in one arm and said balancing networks in sequence in another arm of said bridge circuit, means for storing energy proportional to the unbalance current of said bridge circuit for each balancing network connected thereto, means for comparing the stored energy for the several connected networks, means for selecting on the basis of this comparison one of said networks providing a tolerable balance of the selected line, and for connecting the selected network to the network terminals of said hybrid coil set and simultaneously disconnecting said selected line from said bridge circuit.

KENNETH G. VAN W'YNEN. 

